In radio frequency circuits, such as a receiver or transceiver, a voltage controlled oscillator (VCO) is used as a frequency synthesizer to down-convert or up-convert a radio frequency signal. A VCO may comprise an oscillator designed to be controlled in frequency by a received voltage generated by a VCO control system formed by a frequency divider, a frequency and phase detector, a charge pump and a low pass filter. In the VCO control system, the output of the frequency divider is compared with a reference signal at the frequency and phase detector. The output of the frequency and phase detector is coupled to the low pass filter and further coupled to the oscillator. As a result, the oscillator generates a desired signal in response to the voltage from the low pass filter.
During the process of fabricating semiconductor chips, a plurality of VCO circuits may be built on a wafer. In order to detect defective voltage controlled oscillators of a wafer, various testing circuits are employed to test voltage controlled oscillators during different phases of fabricating semiconductor devices. There are two major types: semiconductor testing performed at wafer level and semiconductor testing performed at packaging level. An advantageous feature of wafer level testing is that wafer level product testing helps to reduce the cost of package and improve the yield.
Wafer level testing can be done by either using a probe card or built-in self-test circuits. A probe card may comprise a variety of probes. Each of the variety of probes may be coupled to a testing pad on a wafer to be tested. The probe card generates a testing signal and reads the testing results from the probes coupled to the testing pads on the wafer. If one circuit block on the wafer does not work or its result is out of the limit to which the circuit block is specified, the probe card can find the failure through the result from the probe coupled to the defective circuit block. By employing the probe card, defective circuit blocks can be found so that they are screened out before the wafer is sent to the next stage of a semiconductor fabrication process. As a result, the cost for packaging defective chips will be saved.
Built-in self-test circuits may be placed at regions wherein scribe lines are drawn. While conventional built-in self-test circuits may only test open-circuit, short-circuit and dc characteristics of an active device, conventional built-in self-test circuits may be not sufficient for evaluating a VCO because the ac characteristics of a VCO is a key factor to decide whether the radio frequency performance of the VCO is within the limit to which the VCO is specified. As a result, despite passing wafer level testing of open-circuit, short-circuit and dc characteristics, some VCO circuits may still fail to pass the final packaged chip test.